1. Field of Invention
This invention relates to power delivery systems for the transduction of mechanical power into acoustic power through the oscillation of an entire resonator to excite a resonant mode, having applications to any acoustic resonator shape.
2. Description of Related Art
There are a number of different ways to deliver power to a standing acoustic wave which are known in the field of acoustics. The method of entire resonator driving, as described in U.S. Pat. Nos. 5,319,938 and 5,515,684, depends on vibrating the entire resonator back and forth in order to use the resonator's inner surface area as the power delivery surface. This approach requires a motor that provides a dynamic force to create the resonator oscillation.
As shown in U.S. Pat. Nos. 5,319,938; 5,231,337; and 5,515,684, incorporated herein by reference, motors used for entire resonator driving typically comprise two moving motor components. FIG. 1 illustrates a prior art device where motor component 4 is rigidly connected to the fluid-filled acoustic resonator 2, and motor component 6 is resiliently mounted to motor component 4 by a spring 8. When a dynamic force is generated between these two motor components, they move dynamically in reactive opposition to each other, thus causing the entire resonator to oscillate so that power is delivered to the fluid. The heavier motor component 6 may be resiliently connected to ground.
FIG. 2 shows a lumped element diagram of the prior art device of FIG. 1. The fluid within the resonator is modeled as spring 14 and mass 12. Associated with each spring is a damper. Since motor mass 4a and resonator mass 2a are rigidly connected they comprise a single moving mass of the system.
Power is delivered to the standing wave according to 1/(2.omega.)FA sin .theta., where .omega.=2.pi.f with f being the drive frequency, F is the magnitude of the force exerted at the face 10 of motor mass 4a, A is the magnitude of the acceleration of motor mass 4a and the resonator mass 2a, and .theta. is the (temporal) phase angle between F and A. The motor must supply not only the force needed to deliver power to the acoustic load but also to directly oscillate motor mass 4a and resonator mass 2a back and forth. The force required to oscillate masses 2a and 4a is not delivered to the acoustic load. However, generating this mass-driving force results in energy losses due to the motor's transduction efficiency and thus reduces the overall efficiency of the power delivery system.
A further source of inefficiency in the prior art system shown in FIGS. 1 and 2 is its limited control of the power factor sin .theta.. If .theta.=90.degree. then the power factor sin .theta.=1. If .theta. assumes values progressively less or greater than 90.degree. then the required motor force increases thus minimizing the energy efficiency of the power delivery system. Adjusting the resonator mass 2a and the motor mass 4a can help tune the power factor toward unity, but structural stiffness and pressure rating requirements for the resonator as well as design requirements for the motor will limit the degree of freedom to make such adjustments.
It is well known in the art of vibrational motors that adjusting the stiffness of spring 8a of FIG.2 in order to tune the mechanical resonance close to the acoustic resonance will reduce the required motor force for a given power delivery. However, this can result in greatly amplified displacements between the moving components which generate excessive noise and higher spring stresses. A control is generally required to keep the drive frequency locked to the acoustic resonance since sound speed changes due to heating and other effects will cause the acoustic resonant frequency to drift during operation. If the mechanical resonance frequency is tuned close to the acoustic resonance, then severe control problems can occur due to resonance repulsion phenomena if the resonant frequency drift brings the two resonant peaks too close together.